Method of manufacture of piezo electric crystals



April 5,1955 1, [MLER 2,705,392

METHOD OF MANUFACTURE OF PIEZO ELECTRIC CRYSTALS Filed June 11, 1952 UGH. BLANK JROUGH LAP ETCH,

MEmuM LAP 0 ETCH FINE LAP JZ -ETCH 5 FINISH LAP I 65-- ETCH 7a V T -7 7 FINAL FINISH INVENTOR Ti:p l Domed OJ United States Patent METHOD OF MANUFACTURE OF PIEZO ELECTRIC CRYSTALS Donald I. Imler, Gettysburg, Pa., assignor t0 Selectronics, Inc., Carlisle, Pa., a corporation of Delaware Application June 11, 1952, Serial No. 292,915

8 Claims. (Cl. 51-283) My invention relates broadly to a method of manufacturing piezoelectric crystals and more particularly to an improved method of processing high frequency crystal blanks.

One of the objects of my invention is to provide a method of manufacuring piezoelectric crystals by which crystals operating above approximately 20 megacycles may be reliably produced on a mass production scale.

Another object of my invention is to provide a process of successive alternate grinding and etching manufacturing steps by which efiicient piezoelectric crystals operating in the range above approximately 20 megacycles are accurately produced.

Other and further objects of my invention reside in a process of producing piezoelectric crystals in the frequency range above 20 megacycles employing successive and differing grinding production stages with intermediate etching stages as set forth more fully in the specification hereinafter following by reference to the accompanying drawings, in which:

Figure 1 is a chart or flow diagram illustrating the successive stages of operation in producing piezocrystals in accordance with the method of my invention; and Figs. 2, 3, 4, 5, 6, and 7 illustrate the dilferent conditions of the piezoelectric plate existing through the several grinding and etching stages to the final finished condition illustrated in Fig. 7.

Crystal control of radio frequency generators operating at frequencies well above 100 megacycles is both practical and desirable. It is diflicult to mass produce quartz crystals for operation above 20 megacycles by conventional methods, and in order to obtain suitable high frequency control, a thicker, lower frequency crystal is processed, and this unit is then operated on one of its mechanical harmonics. Crystals designed to operate on a mechanical harmonic must have surfaces that are flat and uniformly semi-polished.

Conventional methods of processing this type of crystal contemplate the obtaining of an approximately square blank from a quartz bar, by a series of sawing operations. The rough blank is usually about .05 inch thick and about /2 inch square. The blank is then placed in a planetary type lapping machine and the thickness dimension is reduced by the grinding action of a coarse grinding material. After approximately .02 inch of the blank has been removed, the blank is cleaned and the lapping machine charged with a finer abrasive, and the crystal is again lapped. The lapping operations are continued until the desired thickness and finish are obtained. During the final lap a very fine abrasive is used, usually optical powder, and the blank is lapped to a given fundamental frequency. The fundamental frequency is determined by causing the crystal to oscillate in an appropriate electronic oscillator, and the frequency of the oscillations is determined by use of a frequency meter.

The crystal is thus made ready for etching. This operation is necessary before final finish, since it prevents excessive aging. A change in frequency and equivalent resistance over a length of time occurs as attributes of aging and is due primarily to either loss of quartz particles from the surface of the oscillating blank or to the change in characteristics of any material deposited upon the surface of the blank. Under some circumstances the blank has metal electrodes directly applied to its surfaces and over a period of time these metal electrodes change mechanical characteristics, that is either 2,705,392 Patented Apr. 5, 1955 a loss of mass or the regrouping by agglomeration or change in mechanical composition. Crystal blanks of the nonplated type will have their aging effects, due to the first-mentioned surface condition, lessened by appropriate etching. Possible minor aging adjustments will occur due to modifications of the mechanical constants of the quartz itself. Changes in frequency can be effected by, for instance, irradiation with X-ray or high localized temperatures. Hydrofluoric acid is used in the etching bath. The frequency of the crystal after etching is generally higher than desired, and is lowered by plating a thin film of metal on the surfaces, and loading the crystal down to the desired frequency. The crystal can now be operated on the third, fifth, seventh, etc., mechanical harmonic to control the frequency of a high frequency vacuum tube generator.

My invention provides a method of processing crystals which is a radical departure from the foregoing conventional procedure and improves crystal uniformity and activity on overtone operation and in the frequency range of 5096 megacycles and up to megacycles and beyond. I provide for the etching of the crystal surfaces after each laping stage. The process of alternately grinding and etching the blank during fabrication at each step in the crystal production results in a crystal which will be free of interfering surface scratches; and wherein flatness and parallelism of surfaces is more accurately secured; and wherein precise frequency characteristics and tolerances are more faithfully obtained; and wherein the mass laping of high frequency overtone crystals becomes a practical manufacturing operation.

The problem presented here is that of getting crystals to operate in the frequency range 5096 megacycles and up to 100 megacycles and beyond. It is not possible to grind crystal blanks mechanically beyond a thickness that will cause them to operate above 20 megacycles on their fundamental mode of oscillation. Accordingly, the crystal is ground to a certain frequency and then is made to operate on a harmonic. For example, a crystal ground to operate at 8 megacycles and operating on the fifth mechanical harmonic produces a resultant frequency of 40 megacycles. It is very desirable to operate much above 100 megacycles, which can be done if the crystaloperates on the seventh, ninth, or higher harmonies. The point is that in order to have the crystal operate with the necessary activity on these harmonics, the blank must have surfaces that approximate optical flatness and must be uniformly semi-polished.

The conventional mechanical lapping processes that have been and are employed in the crystal industry do not produce a blank having these requisite qualifications. The new processing method of my invention does successfully and practically produce blanks to such requisites.

Using the normal rough lap with a 2F and 600 grain mixed abrasive, the surface of a crystal blank will have irregularities due to the pattern developed on the surface of the blank through the grinding operation. The term 2F refers to an abrasive supplied by the Carborundum Company and is a silicon carbide with an average size of 30 microns and an averate mesh of 500. The term 600 grains refers to an abrasive supplied by Norton Company, often referred to as Crystolon No. 600 and is a silicon carbide abrasive of 12 microns average size and 1250 average mesh. There will also be loose particles of quartz and occluded abrassives on the surface of the blank. The use of the etching process immediately following the rough lap will smooth the irregularities, remove the pattern, and remove the loose particles and abrasives and prepare the surface for the next finer abrasive. Without this, the loose crystal particles and occluded rough abrasive will contaminate the finer grain used in the next laping process and cause scratches and patterns that are undesirable. This is then continued in the following lapping stages and serves the same purpose. The end result is a smooth, flat blank. The use of the intermediate etching has, in actual production of crystals, resulted in a great improvement in the operwere found suitable for acceptance and delivery. However, under the process of my invention the percentage of acceptable units in a production run is better than 90%. My process also makes it feasible to produce on a mass production scale crystals that will operate on the higlller harmonics at frequencies well above 100 megacyc es.

Referring to the drawings in more detail, Fig. l is a flow chart showing the process of my invention.

Figs. 2, 3, 4, 5, 6, and 7 illustrate the condition of the piezoelectric crystal blank in the successive manufacturing stages, represented in the chart of Fig. 1. That is, the rough piezocrystal blank, designated at 1a in Fig. 2, is the rough blank designated in the block labeled Rough blank at 1 in the flow chart of Fig. 1, and is cut from piezoelectric crystal material in a size to provide a thickness dimension enabling a finished crystal formed from the blank to operate in the range at frequencies above 20 megacycles. The rough characteristics of the blank 1 have been sufficiently magnified in Fig. 2 to illustrate the principles involved in the subsequent stages of treatment.

In Fig. 3 I have shown the first stage of preparation of the piezoelectric crystal where the crystal is subjected to a rough lapping operation, designated at 2 in the flow diagram of Fig. 1 by grinding with a 2F and 600 grain mixed abrasive producing the characteristics represented at 20 in Fig. 3. The crystal thus prepared is checked in a frequency measuring circuit to determine its characteristics. The rough lapped crystal blank with the irregularities of surface represented by reference character 2a, including loose particles of quartz and occluded abrasives on the surface thereof, is then plunged into an etching bath designated by the process step in Fig. 1. The etching bath consists of a concentrated solution of ammonium bi-fluoride maintained at room temperature. The etching operation is continued for approximately twenty minutes.

The etching removes portions of the rough lap surface and smooths the irregularities whereupon the crystal blank is subjected to a further medium lap operation using a 600 grain mixed abrasive illustrated by the process step 4 in Fig. l, producing the crystal, blank illustrated at 4a in Fig. 4.

After the medium lap grinding operation which produces the crystal, represented at 4a in Fig. 4, the crystal thus prepared is subjected to a succeeding etching operation indicated by process step 4, in Fig. 1. This etching operation also consists of submerging the crystal, ground as in Fig. 4, for a period of approximately twenty minutes, in a concentrated solution of ammonium bi-fluoride maintained at room temperature. The piezo crystal prepared as in Fig. 4 is then checked for frequency in a frequency measuring circuit.

Upon conclusion of the etching operation designated as step 4b, the crystal is then subjected to a grinding operation using a smaller size abrasive, that is, a 125 grain mixed abrasive designated as a fine lap represented as step 5 in the flow chart of Fig. 1, producing the crystal blank illustrated at 5a in Fig. 5, in which the surface has become increasingly smooth as compared to the previously illustrated surfaces from the rough blank 1a to the surface 2a and the surface 4a. The term 125 grain is used herein to designate a synthetic aluminum oxide manufactured by the Carborundum Company of 12 /2 microns average size and 800 average mesh. Upon completion of the fine lap operation the piezocrystal, having the surface represented as 5a in Fig. 5, is subjected to the etching operation designated as step 517 in the flow chart of Fig. 1 over a period of approximately twenty minutes in a bath of a concentrated solution of ammonium bi-fluoride at room temperature, as heretofore explained. The crystal finished as at 5a is now subjected to a frequency check test in a frequency measuring circuit.

The etched crystal is then subjected to a finish lap operation designated as at 6 in the flow chart of Fig. 1 by grinding, using a 304 grain mixed abrasive and producing a smooth surface appearance, designated at 6a in Fig. 6. Upon completion of the finish lap operation the piezocrystal, having the surface represented as 6a in Fig. 6, is subjected to the etching operation designated as step 61) in the flow chart of Fig. 1 over a period of approximately twenty minutes in a bath of a concentrated solution of ammonium bi-fluoride at room temperature, as heretofore explained. The crystal finished as at 6a is now subjected to a frequency check in a frequency measuring circuit.

The crystal completed in accordance with the finish lap 6 and the etching step 6b is next subjected to a final finish designated at 7 on the flow chart of Fig. 1, repeating the grinding operation with a 125 grain mixed abrasive as heretofore used in the fine lap operation for producing a polished crystal surface, as represented at 7a of Fig. 7. The crystal finished as in Fig. 7 is checked for frequency in a frequency measuring circuit.

The crystal is now ready for mounting, use and operation. By virtue of the alternate grinding and etching treatment stages the crystal blank is progressively reduced in thickness to a dimension necessary for the production of oscillations in the frequency range above 20 megacycles possessing a very high degree of piezoelectric activity. As heretofore pointed out the process of my invention has been found very eflicient in the production of reliable crystals with minimum loss.

While I have described the process of my invention in its preferred embodiments I realize that modifications may be made and I desire it to be understood that no limitations upon my invention are intended other than may be imposed by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States, is as follows:

1. The method of manufacturing piezoelectric quartz crystals which comprises rough lapping a piezoelectric quartz plate proportioned to respond when finish ground to a frequency range above 20 megacycles, etching the rough lapped quartz plate in a bath of concentrated solution of ammonium bi-fluoride at room temperature for a period of approximately twenty minutes, medium lap grinding the quartz plate after the conclusion of said etching process, subjecting said medium lapped quartz plate to a further etching process, again grinding said quartz plate with a fine lap, repeating the etching process and subsequently imparting a finish lap to the quartz plate and thereafter subjecting the ground lapped quartz plate to a final finish for producing a crystal having an operation range above 20 megacycles.

2. The method of manufacturing piezoelectric quartz crystals which comprises producing a rough blank quartz plate proportioned to respond when finish ground to a frequency range above 20 megacycles and subjecting said blank plate to successive alternate stages of grinding and etching for producing a finished crystal having a frequency range above 20 megacycles.

3. The method of manufacturing piezoelectric quartz crystals which comprises producing a rough blank quartz plate proportioned to respond when finish ground to a frequency range above 20 megacycles and progressively grinding and etching said quartz plate in which the grinding operations are effected with abrasive material of differing grain size and in which each of the etching operations'extends over a time period of approximately twenty minutes for producing a finished quartz plate having a frequency range above 20 megacycles.

4. The method of manufacturing piezoelectric quartz crystals which comprises producing a rough blank quartz plate proportioned to respond when finish ground to a frequency range above 20 megacycles, grinding said quartz plate, thereafter etching the quartz plate and successively repeating said alternate grinding and etching operations at least two additional times for producing a finished quartz plate having a frequency range above 20 megacycles.

5. The method of manufacturing piezoelectric quartz crystals which comprises producing a rough blank quartz crystal proportioned to respond when finish ground to a frequency range above 20 megacycles and subjecting said quartz crystal to at least three successive periods of etching spaced by at least three successive periods of grinding.

6. The method of manufacturing piezoelectric quartz crystals which comprises producing a rough blank quartz plate proportioned to respond when finish ground to a frequency range above 20 megacycles and subjecting said quartz plate to at least three successive periods of etching over time periods of approximately twenty minutes each interposed by at least three successive periods of grinding with abrasives which vary in abrasive characteristics.

7. The method of manufacturing piezoelectric quartz crystals which comprises producing a rough blank quartz plate proportioned to respond when finish ground to a frequency range above 20 megacycles and subjecting said quartz plate to at least three different separate periods of grinding with abrasives of the order of approximately 600 grain mixtures during the first two periods of grinding and of the order of an approximately 125 grain mixture during a succeeding period of grinding, each of said periods of grinding being interposed with a period of etching over a time period of approximately twenty minutes each.

8. The method of manufacturing piezoelectric quartz crystals which comprises producing a rough blank quartz plate proportioned to respond when finish ground to a frequency range above 20 megacycles and subjecting said quartz plate to at least three difierent separate periods of grinding with abrasives interposed with a period of etching where each etching period extends over a time interval of approximately 20 minutes, the first and intermediate grinding operations subjecting said quartz plate References Cited in the file of this patent UNITED STATES PATENTS 2,376,219 Winslow May 15, 1945 2,387,142 Fruth Oct. 16, 1945 2,390,404 Walker Dec. 4, 1945 

2. THE METHOD OF MANUFACTURING PIEZOELECTRIC QUARTZ CRYSTALS WHICH COMPRISES PRODUCING A ROUGN BLANK QUARTZ PLATE PROPORTIONED TO RESPOND WHEN FINISH GROUND TO A FREQUENCY RANGE ABOVE 20 MEGACYCLES AND SUBJECTING SAID BLANK PLATE TO SUCCESSIVE ALTERNATE STAGES OF GRINDING AND ETCHING FOR PRODUCING A FINISHED CRYSTAL HAVING A FREQUENCY RANGE ABOVE 20 MEGACYLCES. 